Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus having a first outlet to provide a thermally conditioned fluid with a first flow characteristic to at least part of a sensor beam path, and a second outlet associated with the first outlet and to provide a thermally conditioned fluid with a second flow characteristic, different to the first flow characteristic, adjacent the thermally conditioned fluid from the first outlet.

This application claims priority and benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/376,653, filed on Aug. 24,2010. The content of that application is incorporated herein in itsentirety by reference.

FIELD

The present invention relates to a lithographic apparatus and devicemanufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. In an embodiment, the liquid isdistilled water, although another liquid can be used. An embodiment ofthe present invention will be described with reference to liquid.However, another fluid may be suitable, particularly a wetting fluid, anincompressible fluid and/or a fluid with higher refractive index thanair, desirably a higher refractive index than water. Fluids excludinggases are particularly desirable. The point of this is to enable imagingof smaller features since the exposure radiation will have a shorterwavelength in the liquid. (The effect of the liquid may also be regardedas increasing the effective numerical aperture (NA) of the system andalso increasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein, or a liquid with a nano-particle suspension (e.g. particleswith a maximum dimension of up to 10 nm). The suspended particles may ormay not have a similar or the same refractive index as the liquid inwhich they are suspended. Other liquids which may be suitable include ahydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueoussolution.

Submersing the substrate or substrate and substrate table in a bath ofliquid (see, for example U.S. Pat. No. 4,509,852) means that there is alarge body of liquid that must be accelerated during a scanningexposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the arrangements proposed is for a liquid supply system toprovide liquid on only a localized area of the substrate and in betweenthe final element of the projection system and the substrate using aliquid confinement system (the substrate generally has a larger surfacearea than the final element of the projection system). One way which hasbeen proposed to arrange for this is disclosed in PCT patent applicationpublication no. WO 99/49504. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet after having passed underthe projection system. That is, as the substrate is scanned beneath theelement in a −X direction, liquid is supplied at the +X side of theelement and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet and is taken up onthe other side of the element by outlet which is connected to a lowpressure source. In the illustration of FIG. 2 the liquid is suppliedalong the direction of movement of the substrate relative to the finalelement, though this does not need to be the case. Various orientationsand numbers of in- and out-lets positioned around the final element arepossible, one example is illustrated in FIG. 3 in which four sets of aninlet with an outlet on either side are provided in a regular patternaround the final element. Arrows in liquid supply and liquid recoverydevices indicate the direction of liquid flow.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets oneither side of the projection system PS and is removed by a plurality ofdiscrete outlets arranged radially outwardly of the inlets. The inletsand can be arranged in a plate with a hole in its center and throughwhich the projection beam is projected. Liquid is supplied by one grooveinlet on one side of the projection system PS and removed by a pluralityof discrete outlets on the other side of the projection system PS,causing a flow of a thin film of liquid between the projection system PSand the substrate W. The choice of which combination of inlet andoutlets to use can depend on the direction of movement of the substrateW (the other combination of inlet and outlets being inactive). In thecross-sectional view of FIG. 4, arrows illustrate the direction ofliquid flow in inlets and out of outlets.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, the idea of atwin or dual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

PCT patent application publication WO 2005/064405 discloses an all wetarrangement in which the immersion liquid is unconfined In such a systemthe whole top surface of the substrate is covered in liquid. This may beadvantageous because then the whole top surface of the substrate isexposed to the substantially same conditions. This has an advantage fortemperature control and processing of the substrate. In WO 2005/064405,a liquid supply system provides liquid to the gap between the finalelement of the projection system and the substrate. That liquid isallowed to leak over the remainder of the substrate. A barrier at theedge of a substrate table prevents the liquid from escaping so that itcan be removed from the top surface of the substrate table in acontrolled way. Although such a system improves temperature control andprocessing of the substrate, evaporation of the immersion liquid maystill occur. One way of helping to alleviate that problem is describedin United States patent application publication no. US 2006/0119809. Amember is provided which covers the substrate W in all positions andwhich is arranged to have immersion liquid extending between it and thetop surface of the substrate and/or substrate table which holds thesubstrate.

SUMMARY

In lithography, the measurement of one or more properties, for exampleposition, is often performed by a sensor in which a beam of radiation isprojected by an emitter onto a mark. The beam can be interfered with byanything in its path. That can result in an error being introduced intoa reading, result in a reading not being made, or result in a readingbeing completely wrong.

It is desirable, for example, to reduce or eliminate the risk of errorin sensor readings.

According to an aspect, there is provided a lithographic apparatuscomprising: a sensor comprising an emitter to project a beam ofradiation along a sensor beam path to a mark; a first outlet to providea thermally conditioned fluid with a first flow characteristic to atleast part of the sensor beam path; and a second outlet associated withthe first outlet and to provide a thermally conditioned fluid with asecond flow characteristic, different to the first flow characteristic,adjacent the thermally conditioned fluid from the first outlet.

According to an aspect, there is provided a lithographic apparatuscomprising: a sensor comprising an emitter to project a beam ofradiation along a sensor beam path to a mark; a first outlet to providea turbulent fluid flow along the sensor beam path; and a second outletto provide a laminar fluid flow substantially enclosing the turbulentfluid flow.

According to an aspect, there is provided a lithographic apparatuscomprising: a table arranged to be positioned under a projection system;a fluid supply system to provide a fluid between the projection systemand the table; a sensor to measure a position of the table relative to areference position, the sensor comprising an emitter and a markstructure comprising a mark, the emitter being fixed relative to areference position and the mark being positioned on the table, or theemitter being positioned on the table and the mark being fixed relativeto a reference position; and an outlet to provide a flow of gas to movea droplet of liquid on and/or prevent a droplet of liquid moving ontothe mark structure and/or the emitter.

According to an aspect, there is provided a device manufacturing methodcomprising projecting a patterned beam of radiation onto a substrate,wherein a property is measured by using an emitter to project a beam ofradiation along a sensor beam path to a mark, and wherein a first flowof thermally conditioned fluid with a first characteristic is providedto at least part of the sensor beam path and a second flow of thermallyconditioned fluid is provided with a second flow characteristic,different to the first flow characteristic, adjacent the thermallyconditional fluid from the first outlet.

According to an aspect, there is provided a device manufacturing methodcomprising projecting a patterned beam of radiation through an immersionliquid onto a substrate positioned on a table, wherein a position of thetable relative to a reference position is measured by using an emitterto emit a beam of radiation on to a mark of a mark structure, whereinthe emitter is fixed relative to a reference position and the mark ispositioned on the table or the emitter is positioned on the table andthe mark is fixed relative to a reference position, and wherein a flowof gas is provided to move a droplet of liquid on and/or prevent adroplet of liquid from moving onto the mark structure and/or emitter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts, in cross-section, a barrier member which may be used inan embodiment of the present invention as an immersion liquid supplysystem;

FIG. 6 is a schematic illustration of a sensor, in cross-section,according to an embodiment of the present invention;

FIG. 7 illustrates a sensor, in plan, of an embodiment of the presentinvention;

FIG. 8 illustrates, in cross-section, the position of a sensor in alithographic apparatus according to an embodiment of the presentinvention;

FIG. 9 illustrates, in cross-section, a sensor according to a furtherembodiment of the present invention;

FIG. 10 illustrates, in cross-section, a further embodiment of thepresent invention; and

FIG. 11 illustrates, in plan, the embodiment of FIG. 10.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device. The supportstructure MT holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structureMT can use mechanical, vacuum, electrostatic or other clampingtechniques to hold the patterning device. The support structure MT maybe a frame or a table, for example, which may be fixed or movable asrequired. The support structure MT may ensure that the patterning deviceis at a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more patterning device tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to ascs-outer and a-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section. Similar to the source SO, theilluminator IL may or may not be considered to form part of thelithographic apparatus. For example, the illuminator IL may be anintegral part of the lithographic apparatus or may be a separate entityfrom the lithographic apparatus. In the latter case, the lithographicapparatus may be configured to allow the illuminator IL to be mountedthereon. Optionally, the illuminator IL is detachable and may beseparately provided (for example, by the lithographic apparatusmanufacturer or another supplier).

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Arrangements for providing liquid between a final element of theprojection system PS and the substrate can be classed into two generalcategories. These are the bath type arrangement in which the whole ofthe substrate W and optionally part of the substrate table WT issubmersed in a bath of liquid and the so called localized immersionsystem which uses a liquid supply system in which liquid is onlyprovided to a localized area of the substrate. In the latter category,the space filled by liquid is smaller in plan than the top surface ofthe substrate and the area filled with liquid remains substantiallystationary relative to the projection system PS while the substrate Wmoves underneath that area. A further arrangement, to which anembodiment of the present invention is directed, is the all wet solutionin which the liquid is unconfined. In this arrangement substantially thewhole top surface of the substrate and all or part of the substratetable is covered in immersion liquid. The depth of the liquid coveringat least the substrate is small. The liquid may be a film, such as athin film, of liquid on the substrate. Any of the liquid supply devicesof FIGS. 2-5 may be used in such a system; however, sealing features arenot present, are not activated, are not as efficient as normal or areotherwise ineffective to seal liquid to only the localized area. Fourdifferent types of localized liquid supply systems are illustrated inFIGS. 2-5. The liquid supply systems disclosed in FIGS. 2-4 weredescribed above.

Another arrangement which has been proposed is to provide the liquidsupply system with a liquid confinement member which extends along atleast a part of a boundary of the space between the final element of theprojection system and the substrate table. Such an arrangement isillustrated in FIG. 5. The liquid confinement member is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). A seal is formed between the liquid confinementand the surface of the substrate. In an embodiment, a seal is formedbetween the liquid confinement structure and the surface of thesubstrate and may be a contactless seal such as a gas seal. Such asystem is disclosed in United States patent application publication no.US 2004-0207824.

FIG. 5 schematically depicts a localized liquid supply system with abarrier member 12, IH. The barrier member extends along at least a partof a boundary of the space between the final element of the projectionsystem and the substrate table WT or substrate W. (Please note thatreference in the following text to surface of the substrate W alsorefers in addition or in the alternative to a surface of the substratetable, unless expressly stated otherwise.) The barrier member 12 issubstantially stationary relative to the projection system in the XYplane though there may be some relative movement in the Z direction (inthe direction of the optical axis). In an embodiment, a seal is formedbetween the barrier member and the surface of the substrate W and may bea contactless seal such as a fluid seal, desirably a gas seal.

The barrier member 12 at least partly contains liquid in the space 11between a final element of the projection system PS and the substrate W.A contactless seal 16 to the substrate W may be formed around the imagefield of the projection system so that liquid is confined within thespace between the substrate W surface and the final element of theprojection system PS. The space is at least partly formed by the barriermember 12 positioned below and surrounding the final element of theprojection system PS. Liquid is brought into the space below theprojection system and within the barrier member 12 by liquid inlet 13.The liquid may be removed by liquid outlet 13. The barrier member 12 mayextend a little above the final element of the projection system. Theliquid level rises above the final element so that a buffer of liquid isprovided. In an embodiment, the barrier member 12 has an inner peripherythat at the upper end closely conforms to the shape of the projectionsystem or the final element thereof and may, e.g., be round. At thebottom, the inner periphery closely conforms to the shape of the imagefield, e.g., rectangular, though this need not be the case.

In an embodiment, the liquid is contained in the space 11 by a gas seal16 which, during use, is formed between the bottom of the barrier member12 and the surface of the substrate W. The gas seal is formed by gas,e.g. air or synthetic air but, in an embodiment, N₂ or another inertgas. The gas in the gas seal is provided under pressure via inlet 15 tothe gap between barrier member 12 and substrate W. The gas is extractedvia outlet 14. The overpressure on the gas inlet 15, vacuum level on theoutlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow 16 inwardly that confines the liquid. The forceof the gas on the liquid between the barrier member 12 and the substrateW contains the liquid in a space 11. The inlets/outlets may be annulargrooves which surround the space 11. The annular grooves may becontinuous or discontinuous. The flow of gas 16 is effective to containthe liquid in the space 11. Such a system is disclosed in United Statespatent application publication no. US 2004-0207824.

The example of FIG. 5 is a so called localized area arrangement in whichliquid is only provided to a localized area of the top surface of thesubstrate W at any one time. Other arrangements are possible, includingfluid handling systems which make use of a single phase extractor or atwo phase extractor as disclosed, for example, in United States patentapplication publication no US 2006-0038968. In an embodiment, a singleor two phase extractor may comprise an inlet which is covered in aporous material. In an embodiment of a single phase extractor the porousmaterial is used to separate liquid from gas to enable single-liquidphase liquid extraction. A chamber downstream of the porous material ismaintained at a slight under pressure and is filled with liquid. Theunder pressure in the chamber is such that the meniscuses formed in theholes of the porous material prevent ambient gas from being drawn intothe chamber. However, when the porous surface comes into contact withliquid there is no meniscus to restrict flow and the liquid can flowfreely into the chamber. The porous material has a large number of smallholes, e.g. of diameter in the range of 5 to 300 μm, desirably 5 to 50μm. In an embodiment, the porous material is at least slightlyliquidphilic (e.g., hydrophilic), i.e. having a contact angle of lessthan 90° to the immersion liquid, e.g. water.

Another arrangement which is possible is one which works on a gas dragprinciple. The so-called gas drag principle has been described, forexample, in United States patent application publication nos. US2008-0212046, US 2009-0279060 and US 2009-0279062. In that system theextraction holes are arranged in a shape which desirably has a corner.The corner may be aligned with the stepping or scanning directions. Thisreduces the force on the meniscus between two openings in the surface ofthe fluid handing structure for a given speed in the step or scandirection compared to if the two outlets were aligned perpendicular tothe direction of scan.

Also disclosed in US 2008-0212046 is a gas knife positioned radiallyoutside the main liquid retrieval feature. The gas knife traps anyliquid which gets past the main liquid retrieval feature. Such a gasknife may be present in a so called gas drag principle arrangement (asdisclosed in US 2008-0212046), in a single or two phase extractorarrangement (such as disclosed in Unites States patent applicationpublication no. US 2009-0262318) or any other arrangement.

Many other types of liquid supply system are possible. The presentinvention is not limited to any particular type of liquid supply system.As will be clear from the description below, an embodiment of thepresent invention may use any type of localized liquid supply system. Anembodiment of the invention is particularly relevant to use with anylocalized liquid supply system as the liquid supply system. Furthermore,the invention is not limited to lithographic apparatus in which imagingis done through an immersion fluid or liquid.

Many sensors used in a lithographic apparatus rely on a beam ofradiation passing from an emitter to a mark. The mark may be a sensor,may be a mark in front of a sensor or may be a mark which redirects(e.g., reflects, refracts, diffracts) the beam from the emitter to areceiver. Sensors of this type can have errors and/or inaccuracies as aresult of the sensor beam path which the beam takes from the sensor tothe mark (and onwards) not being of a completely uniform material. Forexample, dust in the sensor beam path, liquid from, for example, animmersion fluid supply system, or fluid (e.g. gas) with varyingtemperature and/or composition and/or humidity can all result ininaccuracy and/or error in the reading of the sensor. In the case ofnon-uniform fluid, a variation in composition of the fluid and/ortemperature and/or humidity can result in a change of refractive indexand thereby introduce an error or inaccuracy into the reading of thesensor.

Fluid between an emitter and a mark (and between the mark and thereceiver) may be conditioned to reduce or minimize changes of therefractive index of the fluid through which the sensor beam passes. Inone example, a turbulent flow of gas is used to mix the temperatureconditioned gas. In this way, a local difference in temperature,humidity or pressure is divided over the entire volume of theconditioned gas (along the length of the sensor beam path), resulting insubstantially constant gas properties over the entire length of thesensor beam path.

A disadvantage of turbulent flow is that it also mixes in unconditionedgas from the environment adjacent the sensor. This unconditioned gas canthen find its way into the sensor beam path resulting in error. This isbecause the unconditioned gas may be of a different temperature,humidity and/or composition (of gases) or may contain particles. Themixture of the unconditioned gas with the conditioned gas can have anegative effect on the stability of the refractive index of the gas inthe sensor beam path. This reduces the accuracy of the sensor.

A turbulent flow results in a high uniformity of refractive index in thebeam path of the sensor. A fundamental drawback of a turbulent flow isthe high amount of unconditioned gas that is entrained in theturbulence, leading to low absolute refractive index stability.

A high stability of refractive index can be obtained by making a laminarflow of gas in the sensor beam path. A laminar flow of gas does notsubstantially entrain unconditioned gas. This therefore leads to lowsensor noise and high stability at rest. However, during movement of acomponent of the sensor relative to another component of the sensor(such as in some sensors used in a lithographic projection apparatus), alaminar flow can create a high refractive index gradient(non-uniformity) which can lead to systematic measurement errors.

An embodiment of the present invention combines the low static noiseproperties of a laminar flow with high dynamic mixing properties ofturbulent flow.

An embodiment of the present invention is described below with referenceto a sensor such as that described in United States patent applicationpublication no. US 2010/0157263, hereby incorporated in its entirety byreference, although an embodiment of the invention can be applied to anysensor in which an emitter projects a beam of radiation to a mark. Sucha sensor comprises an emitter 20 to project a beam B of radiation onto amark 30. The mark 30 redirects the beam B of radiation projected by theemitter 20 to a receiver 40 which, in an embodiment, is positioned nextto the emitter 20. The path which the beam of radiation B (the sensorbeam path illustrated in dashed lines) is conditioned by a flow of fluidillustrated by arrows drawn in solid lines in FIG. 6. The descriptionbelow assumes the fluid to be gas but could equally well be liquid.

In FIG. 6 the emitter 20 and receiver 40 are mounted on a substratetable WT. The mark 30 is mounted in fixed relation relative to areference position, for example relative to the projection system PS ofthe lithographic apparatus as illustrated in FIG. 8. The emitter 20 andreceiver 40 may be mounted in a fixed relation relative to the referenceposition and the mark 30 may be mounted on the substrate table WT asillustrated, for example in FIGS. 9-11. Additionally the sensor may beused to measure the position of a table other than a substrate table(for example a measurement table which carries one or more sensors anddoes not support a substrate as does a substrate table WT).Alternatively, the sensor may be used to measure the position of thesupport structure MT to support the patterning device MA. Furthermore,the an embodiment of the present invention may be applied to other sortsof sensors. For example, the an embodiment of the invention may beapplied to an interferomic sensor system which measures the position ofa table, for example the substrate table or another object.Additionally, an embodiment of the invention may be applied to othertypes of sensors including dose sensors, SMASH sensors (aself-referencing interferometer used for alignment with a symmetricalmark, for example as described in European patent applicationpublication no. EP-A-1,148,390, hereby incorporated in its entirety byreference), transmission image sensors, alignment sensors and levelsensors. An embodiment of the present invention is suited for a sensorin which the emitter 20 and mark 30 and/or the receiver 40 and mark 30are separated only by a few centimeters, for example by 10 cm or less,desirably by 5 cm or less.

The sensor of FIG. 6 measures the position of the substrate table WTrelative to the mark 30 in the XY plane (the plane of a top surface ofthe substrate table WT) as well as in the orthogonal Z direction (thedirection of the optical axis of the apparatus). The sensor measures theposition to subnanometer accuracy. Due to the working principle of thesensor, the sensitivity to refractive index variation is different forthe X and Y directions compared to the Z direction. The XY accuracy ofthe sensor depends upon a high refractive index uniformity along thesensor beam path whereas Z accuracy demands a high absolute refractiveindex stability.

Thermally conditioned gas is provided by a source 50. Because theemitter 20 and receiver 40 are provided on a moving object, the gas flowshould maintain uniformity and stability both during movement of thesubstrate table WT at up to 2 m/s relative to the grid 30 as well as atstandstill. Additionally, the gas flow should be robust against othergas disturbances in the lithographic apparatus.

FIG. 6 discloses a lithographic apparatus comprising a sensor, a firstoutlet 110 and a second outlet 120. The sensor comprises an emitter 20to project a beam B of radiation along a sensor beam path to a mark 30.The first outlet 110 provides a turbulent fluid flow along the sensorbeam path. The second outlet 120 provides a laminar fluid flowsubstantially enclosing the turbulent fluid flow.

The first outlet 110 may be provided adjacent the emitter 20 andreceiver 40. The first outlet 110 is arranged to provide a gas from thesource 50. On exiting the outlet 110, the gas moves to the sensor beampath. The gas may be a mixture of gases or may be a single gas, forexample an inert gas such as nitrogen. The gas may be conditioned interms of its temperature, its humidity and/or composition. The firstoutlet 110 may be constructed to provide a thermally conditioned gas.The first outlet 110 may be constructed to provide gas with a first flowcharacteristic. The first outlet 110 is configured to provide the firstfluid as turbulent flow. For example, the first flow characteristic maybe a Reynolds number of greater than 4000, desirably greater than 8000.

The first outlet 110 may be a single continuous outlet or a series ofoutlets such as discrete outlets in a line. The first outlet 110 isconfigured such that the gas exiting it surrounds the sensor beam pathand/or flows into the sensor beam path. In one embodiment the firstoutlet 110 surrounds the emitter 20 and/or the receiver 40, asillustrated in FIG. 7.

The second outlet 120 is provided. The second outlet 120 may beassociated with the first outlet 110. The second outlet 120 isconstructed and arranged to provide a flow of gas from the source 50.The gas from the second outlet 120 may be provided adjacent the gas fromthe first outlet 110. The second outlet 120 may be constructed andconfigured to provide a thermally conditioned gas. The second outlet 120may be constructed and configured to provide gas with a second flowcharacteristic. The second flow characteristic may be different from thefirst flow characteristic. The second outlet 120 is configured toprovide the gas with the second flow characteristic as a laminar flow.For example, the second outlet is configured to provide the gas with aReynolds number of less than 2300, desirably less than 2000. The flow ofgas out of the second outlet 120 acts as a shield to substantiallyprevent non-conditioned gas radially outwardly of the second outlet 120with respect to an optical axis of the sensor from being entrained bythe turbulent flow of the gas exiting the first outlet 110. Thus thesecond outlet 120 is configured such that the flow of the second gassubstantially prevents non-conditioned gas reaching the sensor beampath. Non-conditioned gas may have particles, a composition, atemperature and/or an amount of humidity that may disturb the beam B.

The second outlet 120 is on a side of the first outlet 110 opposite to aside of the first outlet 110 on which the emitter 20 and receiver 40 areprovided.

As can be seen in FIG. 7, both the first outlet 110 and second outlet120 surround the emitter 20 and receiver 40. The second outlet 120 maysurround the first outlet 110. The first and second outlets 110, 120 maybe concentric. The emitter 20 and receiver 40 are positioned within thefirst outlet 110.

The first and second outlets 110, 120 provide gas flow as illustrated inFIG. 6. The gas flow is in a direction substantially parallel to adirection of the sensor beam path. That is, the flow is towards the mark30. The inner flow through the first outlet 110 is turbulent. Theturbulent flow entrains fluid from the laminar flow exiting the secondoutlet 120. This helps ensure good dynamic mixing properties when thesubstrate table WT is moving at high speed. This helps result in lowsystematic XY position measurement error. Additionally it helps solvethe problem of entrapment of unconditioned gas by the turbulent flow byactively supplying gas just outside of the turbulent flow. The laminarflow essentially acts as a seal, by blowing away any unconditioned gasthat is about to enter the conditioned gas volume. The unconditioned gasstays outside of the laminar flow and is substantially prevented fromentering the conditioned volume.

A significant reduction in the amount of unconditioned gas inside thevolume defined between the mark 30, the substrate table WT and the firstoutlet 110 is achievable. Where only the first outlet 110 is provided,that volume may contain 51%, 54% and 62% unconditioned gas for scanspeeds of 0, 0.7 and 1.4 m/s respectively. In contrast, with theprovision of the second outlet 120 providing a laminar flow, thepercentage of unconditioned gas in the space may fall to 0%, 2% and 5-6%for movement speeds of 0, 0.7 and 1.4 m/s respectively. This shows asignificant improvement and leads to a three times reduction in noise inthe measurement results in all directions of measurement.

A radial inward flow of the gas exiting the first and second outlets110, 120 is achieved by providing an inlet 130 radially outwardly of theemitter 20 and receiver 40 and radially inwardly of the first outlet110. That is, the inlet 130 is provided on the same side of the sensoras the emitter 20 and receiver 40 (as opposed to being provided adjacentthe mark 30). Additionally, the inlet 130 is provided on the same sideof the sensor as the first and second outlets 110, 120.

In order to achieve the different flow characteristics of fluids exitingthe first and second outlets 110, 120, the cross sectional areas of thefirst and second outlets 110, 120 may be different as illustrated,assuming that both outlets 110, 120 are attached to the same source andwith the same pressure. In this way, the gas exiting the second outlet120 has a lower velocity than that exiting the first outlet 110 therebyachieving laminar flow. The turbulent flow is achieved by the high exitvelocity of the gas exiting the first outlet 110. Other ways ofachieving the different flow characteristics are possible such asproviding the gas out of the first and second outlets 110, 120 atdifferent pressures, or by encouraging turbulent flow by placing flowrestrictions or obstacles in the first outlet 110.

As with the first outlet 110, the second outlet 120 and inlet 130 maycomprise one or more openings. The first and second outlets 110, 120 andthe inlet 130 face the mark and/or are in a surface in which the emitter20 is mounted.

In FIG. 6 the gas flows out of the outlets 110, 120 are shown as beingperpendicular to the surface in which the outlets 110, 120 are formed.This is not necessarily the case and the flows could be angled radiallyinwardly towards the sensor beam path, for example. This could beachieved, for example, by angling the conduits which provide gas to theoutlets 110, 120 away from being perpendicular to the plane of thesurface.

Instead of nitrogen, argon or any gas or mixture of gases or even liquidwhich is compatible with the apparatus and has a substantially constantrefractive index at a given temperature can be used. As explained above,the combination of first and second outlets 110, 120 with gas exitingwith a different flow characteristic can be used on other types ofsensor. For example, a different type of sensor may replace a reflectivemark 30 with a transmissive mark and provide the receiver 40 on theother side of the mark 30 to the emitter 20. The same principles asdescribed herein could be applied to such a system in which a turbulentflow is provided radially inwardly of a laminar flow to preventunconditioned gas being sucked into the sensor beam path by theturbulent flow.

FIG. 8 shows, in cross-section, how the components of FIG. 6 may beassembled into a lithographic apparatus. In FIG. 8 the substrate tableWT is positioned under the projection system PS and immersion fluidsupply system 12. The emitter 20 and receiver 40 are positioned at anedge of the substrate table WT. The beam B from the emitter 20 isprojected towards a mark 30 of a mark structure 35 which is held abovethe substrate table WT in a known position relative to the projectionsystem PS. For example the mark 30 may be held by the reference frameRF. The mark 30 surrounds, in plan, the projection system PS and thesubstrate table WT is provided with at least three emitter 20/receiver40 combinations such that the position of the substrate table WTrelative to the mark 30 and thereby the projection system PS can bemeasured as is described in United States patent application publicationno. US 2010/0157263. The mark 30 is formed on a surface of the markstructure 35 which faces away from the emitter 20/receiver 40.Therefore, the beam B from the emitter 20 passes through the markstructure 35 (which may be a plane parallel plate of quartz, forexample) before impinging on the mark 30 and being redirected backthrough the mark structure 35 and then through the atmosphere to thereceiver 40. Any droplets or contamination on the surface of the markstructure 35 facing the emitter 20/receiver 40 could interfere with thebeam B thereby resulting in erroneous measurement and/or no measurementat all. The principles described elsewhere apply equally to such anarrangement as illustrated in FIG. 8 as well as to the otherarrangements illustrated where the mark 30 is on a surface facing theemitter 20/receiver 40.

In FIG. 9 the mark 30 is provided on the substrate table WT and theemitter 20/receiver 40 combination is provided is fixed relative to theprojection system PS. Otherwise the embodiments of FIGS. 8 and 9 are thesame. In order to keep the footprint, in plan, of the substrate table WTto a reasonable size, the mark 30 is provided around an edge of asubstrate table WT. The size of the mark 30 is not large enough so thatonly a single emitter 20/receiver 40 combination can be used. Thereforea plurality of emitter 20/receiver 40 combinations are provided andthose under which the mark 30 is at any given time can be used to make areading of the position of a substrate table WT. Such a system isdescribed, for example, in United States patent application publicationno. US 2007/0288121, which is hereby incorporated in its entirety byreference. FIG. 11, shows in plan, an arrangement of emitter 20/receiver40 combinations.

A difficulty with providing the mark 30 on the substrate table WT isthat the immersion liquid supply system 12 will at a certain moment needto cross the mark 30, for example during substrate table swap under theprojection system PS. This can result in immersion liquid being leftbehind upon the mark 30. Such immersion liquid left behind on the mark30 can be in the sensor path and thereby interfere with the reading madeby the sensor. This can lead to errors in the sensor system. The use ofgas flows as illustrated in FIG. 9 can be used to move a droplet on themark 30 out of the sensor beam path. This is an additional advantage tothat already described above with reference to FIG. 6.

FIG. 10 shows a cross section of a further embodiment which is the sameas that shown in FIG. 9 except as described below. Instead of providingfirst and second outlets 110, 120, in FIG. 10 two examples of possiblealternative outlets 180 are illustrated. The alternative outlets 180provide a flow of gas for the movement of a droplet of liquid on themark 30. In one embodiment the outlet 180 is provided in a surface inwhich the emitter 20/receiver 40 is provided. In a further embodimentthe additional outlet 180 is provided on the substrate table WT. Theoutlet 180 may be configured to direct the flow of gas at an angle otherthan 90° to the surface of the mark 30. This may be more effective inmoving liquid on the mark 30. However a flow of gas perpendicular indirection to the surface of the mark 30 will also be effective. Forexample, relative movement of the substrate table WT relative to theflow may be effective to move a droplet of liquid on the mark 30 out ofthe sensor beam path.

A further embodiment is illustrated in FIG. 11 which shows a systemsimilar to that of FIG. 9 in plan. However only a single outlet 200 isprovided to direct a flow of gas towards the mark 30. The single outlet200 can surround one or more combinations of emitter 20/receiver 40though this is not necessarily the case. For example, each emitter20/receiver 40 could be provided with an individual outlet 200. In oneembodiment the flow of gas through the outlet 180/200 is switchable sothat the flow of gas only occurs when the mark 30 is under theassociated emitter 20/receiver 40 combination. The flow of gas out ofthe outlet 200 provides a barrier (for example a gas curtain) which doesnot allow droplets to pass or moves a droplet already present out of theway. Therefore, the flow of gas out of outlet 200 is to move a dropletof liquid and/or prevent a droplet of liquid moving onto the mark 30.

In an embodiment, there is provided a lithographic apparatus comprisinga sensor, a first outlet and a second outlet. The sensor comprises anemitter to project a beam of radiation along a sensor beam path to amark. The first outlet provides a turbulent fluid flow along the sensorbeam path. The second outlet provides a laminar fluid flow substantiallyenclosing the turbulent fluid flow. The turbulent and/or laminar flowmay be thermally conditioned.

The second outlet may be on a side of the first outlet opposite to aside of the first outlet on which the emitter and/or mark is provided.

The turbulent fluid flow may have a Reynolds number of greater than4000. The laminar fluid flow may have a Reynolds number of less than2300.

The second outlet may be configured such that the laminar fluid flowsubstantially prevents non-conditioned fluid from reaching the sensorbeam path.

The first and second outlets may be configured to provide fluid in adirection substantially parallel to a direction of propagation of thebeam.

The first outlet may be configured such that the turbulent fluid flowsurrounds the sensor beam path.

The first outlet may surround the emitter and/or the mark. The firstoutlet may comprise one or more openings. The second outlet may surroundthe first outlet. The second outlet may comprise one or more openings.

The sensor may further comprise a receiver and the mark may reflect thebeam of radiation from the emitter to the receiver.

The lithographic apparatus may further comprise a table to support asubstrate. The sensor may be configured to measure a position of thetable relative to a reference point.

The emitter or the mark may be mounted on the table. The other of theemitter and the mark may be mounted in fixed relation relative to thereference point.

The lithographic apparatus may further comprise an immersion fluidsupply system configured to provide immersion fluid between a projectionsystem of the lithographic apparatus and a substrate. The first outletand/or the second outlet may be configured such that fluid exiting thefirst and/or second outlet is effective to move a droplet on the markand/or the emitter such that the droplet does not interfere with thebeam of radiation. The fluid may be a gas.

In an embodiment, there is provided a lithographic apparatus comprisinga table, a fluid supply and a sensor. The table is arranged to bepositioned under a projection system. The fluid supply system isconfigured to provide a fluid between the projection system and thetable. The sensor is configured to measure a position of the tablerelative to a reference position. The sensor comprises an emitter and amark structure comprising a mark. The emitter is fixed relative to areference position and the mark is positioned on the table, or theemitter is positioned on the table and the mark is fixed relative to areference position. The lithographic apparatus further comprises anoutlet to provide a flow of gas to move a droplet of liquid on and/orprevent a droplet of liquid moving onto the mark structure and/or theemitter. The flow of gas may move a droplet from the mark structure orthe emitter. Alternatively or additionally, the flow of gas may preventa droplet moving onto the mark structure or the emitter.

In an embodiment, there is provided a device manufacturing methodcomprising projecting a patterned beam of radiation onto a substrate;measuring a property using an emitter to project a beam of radiationalong a sensor beam path to a mark; providing a first flow of thermallyconditioned fluid with a first characteristic to at least part of thesensor beam path; and providing a second flow of thermally conditionedfluid with a second flow characteristic. The second flow characteristicis different to the first flow characteristic and is provided adjacentthe thermally conditioned fluid from the first outlet.

In an embodiment, there is provided a device manufacturing method,comprising: projecting a patterned beam of radiation through animmersion liquid onto a substrate positioned on a table; measuring aposition of the table relative to a reference position using an emitterto emit a beam of radiation on to a mark of a mark structure. Theemitter is fixed relative to a reference position and the mark ispositioned on the table or the emitter is positioned on the table andthe mark is fixed relative to a reference position. The method furthercomprises providing a flow of gas to move a droplet of liquid on and/orprevent a droplet of liquid from moving onto the mark structure and/oremitter. The flow of gas may move a droplet from the mark structure orthe emitter. Alternatively or additionally, the flow of gas may preventa droplet moving onto the mark structure or the emitter.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications in manufacturing components with microscale, or evennanoscale features, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the embodiments of the invention maytake the form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g. semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein. Further, themachine readable instruction may be embodied in two or more computerprograms. The two or more computer programs may be stored on one or moredifferent memories and/or data storage media.

The controllers described above may have any suitable configuration forreceiving, processing, and sending signals. For example, each controllermay include one or more processors for executing the computer programsthat include machine-readable instructions for the methods describedabove. The controllers may also include data storage medium for storingsuch computer programs, and/or hardware to receive such medium.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above, whether the immersion liquid is provided in the form ofa bath, only on a localized surface area of the substrate, or isunconfined on the substrate and/or substrate table. In an unconfinedarrangement, the immersion liquid may flow over the surface of thesubstrate and/or substrate table so that substantially the entireuncovered surface of the substrate table and/or substrate is wetted. Insuch an unconfined immersion system, the liquid supply system may notconfine the immersion liquid or it may provide a proportion of immersionliquid confinement, but not substantially complete confinement of theimmersion liquid.

A liquid supply system as contemplated herein should be broadlyconstrued. In certain embodiments, it may be a mechanism or combinationof structures that provides a liquid to a space between the projectionsystem and the substrate and/or substrate table. It may comprise acombination of one or more structures, one or more liquid inlets, one ormore gas inlets, one or more gas outlets, and/or one or more liquidoutlets that provide liquid to the space. In an embodiment, a surface ofthe space may be a portion of the substrate and/or substrate table, or asurface of the space may completely cover a surface of the substrateand/or substrate table, or the space may envelop the substrate and/orsubstrate table. The liquid supply system may optionally further includeone or more elements to control the position, quantity, quality, shape,flow rate or any other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: a sensor comprising an emitterto project a beam of radiation along a sensor beam path to a mark; afirst outlet to provide a turbulent fluid flow along the sensor beampath; and a second outlet to provide a laminar fluid flow substantiallyenclosing the turbulent fluid flow.
 2. The lithographic apparatus ofclaim 1, wherein the turbulent flow, or the laminar flow, or both, isthermally conditioned.
 3. The lithographic apparatus of claim 2, whereinthe second outlet is configured such that the laminar fluid flowsubstantially prevents non-conditioned fluid from reaching the sensorbeam path.
 4. The lithographic apparatus of claim 1, wherein the secondoutlet is on a side of the first outlet opposite to a side of the firstoutlet on which the emitter and/or mark is provided.
 5. The lithographicapparatus of claim 1, wherein the turbulent fluid flow has a Reynoldsnumber of greater than
 4000. 6. The lithographic apparatus of claim 1,wherein the laminar fluid flow has a Reynolds number of less than 2300.7. The lithographic apparatus of claim 1, wherein the first and secondoutlets are configured to provide fluid in a direction substantiallyparallel to a direction of propagation of the beam.
 8. The lithographicapparatus of claim 1, wherein the first outlet is configured such thatthe turbulent fluid flow surrounds the sensor beam path.
 9. Thelithographic apparatus of claim 1, wherein the first outlet surroundsthe emitter, or the mark, or both.
 10. The lithographic apparatus ofclaim 1, wherein the first outlet comprises one or more openings. 11.The lithographic apparatus of claim 1, wherein the second outletsurrounds the first outlet.
 12. The lithographic apparatus of claim 1,wherein the second outlet comprises one or more openings.
 13. Thelithographic apparatus of claim 1, wherein the sensor further comprisesa receiver and the mark reflects the beam of radiation from the emitterto the receiver.
 14. The lithographic apparatus of claim 1, furthercomprising a table to support a substrate and wherein the sensor isconfigured to measure a position of the table relative to a referencepoint.
 15. The lithographic apparatus of claim 14, wherein the emitteror the mark is mounted on the table, and wherein the other of theemitter or the mark is mounted in fixed relation relative to thereference point.
 16. The lithographic apparatus of claim 1, furthercomprising an immersion fluid supply system configured to provideimmersion fluid between a projection system of the lithographicapparatus and a substrate, wherein the first outlet, or the secondoutlet, or both, are configured such that fluid exiting the first and/orsecond outlet is effective to move a droplet on the mark, or theemitter, or both, such that the droplet does not interfere with the beamof radiation.
 17. The lithographic apparatus of claim 1, wherein thefluid is a gas.
 18. A lithographic apparatus comprising: a tablearranged to be positioned under a projection system; a fluid supplysystem to provide a fluid between the projection system and the table; asensor to measure a position of the table relative to a referenceposition, the sensor comprising an emitter and a mark structurecomprising a mark, the emitter being fixed relative to a referenceposition and the mark being positioned on the table, or the emitterbeing positioned on the table and the mark being fixed relative to areference position; and an outlet to provide a flow of gas to move adroplet of liquid on and/or prevent a droplet of liquid moving onto themark structure and/or the emitter.
 19. A device manufacturing method,comprising: projecting a patterned beam of radiation onto a substrate;measuring a property using an emitter to project a beam of radiationalong a sensor beam path to a mark; providing a first flow of thermallyconditioned fluid with a first characteristic to at least part of thesensor beam path; and providing a second flow of thermally conditionedfluid with a second flow characteristic, different to the first flowcharacteristic, adjacent the thermally conditioned fluid from the firstoutlet.
 20. A device manufacturing method, comprising: projecting apatterned beam of radiation through an immersion liquid onto a substratepositioned on a table; measuring a position of the table relative to areference position using an emitter to emit a beam of radiation on to amark of a mark structure, wherein the emitter is fixed relative to areference position and the mark is positioned on the table or theemitter is positioned on the table and the mark is fixed relative to areference position; providing a flow of gas to move a droplet of liquidon and/or prevent a droplet of liquid from moving onto the markstructure and/or emitter.